Hi, yes I'd stick to hay and water for a few days - it'll do no harm.
I found one of mine (they are siblings) had a very sensitive tum when he was young just one or two more pellets or a square inch more of cabbage would produce excess caceotrophes. They are 20 months now and he can eat lots more veg without a problem.
It could be the quantity of veg or a particular veg. I would wait until he has had a good few days of no squishy poops, even a week, and then introduce one veg at a time a tiny bit at a time (I'd start with celery, maybe 1" cut in to slices) continue with the same amount for a few days if no change before trying another veg. Perhaps try some herbs, corinander, parsley, mint or some rocket rather than kale/cabbage or spring greens.
You should be able to ask your vet for a little pot of Avipro or you can get it online I think - Vet UK ?